Systems for controlling web tension in printing presses have traditionally been designed only to regulate or control web tension during normal running conditions. Generally, web is removed from a large, heavy reel of newsprint by means of belt-like friction means that are in contact with the surface of the reel and are frequently referred to as surface-driven. After leaving the reel, the web is usually, at some point prior tp printing, entrained about a biasing roller, such as a dancer roll, the purpose of the dancer roll being to permit regulation of the tension in the web by means of appropriate switching and fluid pressure regulators. Controlling tension is necessary, since it varies greatly during printing operations due to change in the weight, speed and diameter of the reel as web is being payed out. Web tension controls utilized heretofore have frequently included not only means to vary the fluid pressure (usually pneumatic) utilized at the dancer roll but to also control braking means to retard rotation of the paper reel.
Typical of prior art devices used for web tension control is that shown in U.S. Pat. No. 3,913,900 where a pendulum or dancer roll 4 is utilized. In this device, change in tension in web 1 causes roll 4 to pivot, and thereby change the setting of potentiometer 15. The potentiometer controls the speed of drive motor 5 to vary the speed of the web and thereby control web tension.
Another example of web tension control is shown in U.S. Pat. No. 3,202,376. Here a sensing roller 37 pivots in response to the tension in loop 38 to effect change in the adjustment means 45, thereby altering the friction between roll 10 and belts 21-23. Increase or decrease in the frictional engagement between roll 10 and belts 21-23 either increases or decreases the rotative speed of the rolls to vary the tension in the web in direct proportion thereto.
Additional examples of mechanisms or systems to control web tension in strands being payed from a supply roll may be found by referring to U.S. Pat. Nos. 3,813,052; 3,811,637; 4,147,516; 1,978,073 and 2,222,462.
A review of all the patents noted above will show that, as already stated, the prior art displays a variety of mechanisms for controlling the tension of webs during the usual printing operation. What the existing art does not indicate is the importance of being able to effectively control web tension when the web is being threaded around the various rolls prior to press start up. During this threading operation, one man is typically required to control the large, high inertia paper roll, in order to keep the web slack and avoid breakage until the web is fully in the press and ready for tension. In addition, once the web is installed, the tension set on the web is usually decreased until the press and web are running well, then readjusted, back up to the original value. All of these operations require a great deal of manual intervention during the busy start up period.